This invention relates to a pilot assembly for a hydraulic pump. A typical hydraulic pump includes a hydraulic cylinder, a piston which is reciprocable within the hydraulic cylinder, a source of pressurized hydraulic fluid for driving the piston, and a valve for controlling the flow of pressurized hydraulic fluid to the cylinder. The invention finds particular utility with hydraulic pumps used in concrete pumping machines, and the invention will be explained in conjunction with concrete pumps. However, it will be understood that the invention is not limited to concrete pumps but can be used with any hydraulic pump in which pilot pressure is used to switch the main hydraulic valve of the pump.
A concrete pump includes one or more pumping cylinders for pumping concrete and a hydraulic cylinder for each pumping cylinder for driving the piston in the pumping cylinder. Each hydraulic cylinder includes a piston which is extended and retracted during the pumping cycle by hydraulic fluid, and the piston in the hydraulic cylinder drives the piston in the pumping cylinder by means of a rigid connecting rod or the like.
Most concrete pumps include a pair of pumping cylinders and a pair of hydraulic cylinders. The hydraulic cylinders are conventionally synchronized for tandem operation so that one piston is being retracted while the other piston is being extended. This synchronization can be achieved by a slave system which includes a crossover pipe connecting the forward ends of the hydraulic cylinders. As one of the pistons is being extended toward the forward end of the cylinder by pressurized hydraulic fluid, the hydraulic fluid between the rod end of the piston and the forward end of the cylinder, i.e., slave fluid, is forced through the crossover pipe into the other hydraulic cylinder to force the second piston rearwardly. Conversely, when pressurized hydraulic fluid is pumped into the other cylinder to extend the second piston, the second piston forces the slave fluid through the crossover to retract the first piston.
A concrete pump with two hydraulic cylinders generally includes a main 4-way valve which is movable between two positions for alternately supplying pressurized hydraulic fluid to one hydraulic cylinder and then the other. When the main valve is supplying pressurized hydraulic fluid to one cylinder to extend the piston in that cylinder, the valve permits the hydraulic fluid in the other cylinder to pass through the valve to the fluid reservoir as the piston in that cylinder is retracted by the slave fluid.
The main valve is switched back and forth between its two operating positions by a pilot system which can be either electrical or hydraulic. The pilot system switches the main valve when the piston in either of the cylinders reaches its fully extended position.
A concrete pump with only a single hydraulic cylinder also includes a main valve which is movable between two positions. In one position pressurized hydraulic fluid flows through the valve into one end of the cylinder to extend the piston, and hydraulic fluid which is forced out of the cylinder by the extending piston flows through the valve to the fluid reservoir. In the other position the valve directs pressurized hydraulic fluid to the other end of the cylinder to retract the piston, and hydraulic fluid which is forced out of the first end of the cylinder by the retracting piston returns to the fluid reservoir through the valve. The valve is also operated by a pilot system.
Pilot systems which have heretofore been used for switching the main valve usually include some movable element which is engageable by the hydraulic piston or connecting rod as the hydraulic piston reaches the limit of its movement in one direction. Such a movable element can take the form of a travel rod, pin, roller, spring, spool, etc., and the movable element is usually placed in the head of the hydraulic cylinder or at least closely adjacent the hydraulic cylinder. A major problem with such a pilot system is that the pilot system is not readily accessible for inspection, repair, or replacement, and a pilot system which must be moved by the piston or piston rod can cause damage in the event of jams or the like.